The field of the invention generally relates to a method and apparatus for bending tubes such as for making heat exchangers, and more particularly relates to a split bend die and method of rotating the split sections of the bend die after bending a tube to reduce bend angles and vertical spacings between adjacent parallel segments of a tubular heat exchanger.
As is well known, residential furnaces have been constructed using tubular heat exchangers instead of the more conventional clam-shell heat exchangers. With such arrangement, a plurality of stainless steel or aluminized steel tubes are arranged within a heat exchange chamber of a furnace, and one end of each is fired by an individual burner. The hot combustion gases pass through the tubes, and heat is transferred to household return air that is forced across outside surfaces of the tubes.
In the above-described furnace arrangement, it is desirable to maximize the heat exchange surface area within the confined or restricted volume inside the heat exchange chamber. It may also be desirable to minimize the size, and in particular, the height of the heat exchange chamber so that the furnace can be used at installations that have height restrictions. Accordingly, tubes have been bent into serpentine configurations with parallel straight segments to increase the length of tubes that will fit into a heat exchange chamber. In particular, tubes have been rotated between successive bends so that the parallel straight segments are not linearly aligned. Thus, the bends can be seen to zigzag back and fourth when the parallel segments are viewed from their ends. The zigzagging is desirable because it promotes turbulence in household return air that is forced across the outside surfaces of the tubes. Thus, heat transfer is enhanced.
Another reason for zigzagging relates to the apparatus used to bend the tubes. In particular, one apparatus is described in U.S Pat. No. 5,142,895. A tube is seated in the groove of a rotary bend die, and a pressure die and clamp die are moved up against the opposite side of the tube. The bend die and the clamp die are then rotated approximately 180.degree. about a vertical axis while the pressure die moves forward linearly carrying the tube tangentially to the bend point. The clamp die and pressure die are then retracted and returned to their respective initial positions, and the tube is repositioned with respect to the bend die so that another 180.degree. bend can be made. The tube is also rotated to elevate the just formed segment above the path used by the clamp die on the next bend. This tube rotation leads to segments that zigzag rather than being disposed in a single plane. The apparatus used to split bend die wherein an upper section was elevated from a lower section to remove the tube which had been formed with controlled wrinkles past the 180.degree. tangent point.
A later tube bending improvement is described in U.S. Pat. No. 5,284,041 filed May 10, 1993, which is hereby incorporated by reference. After bending the tube, the upper section of the bend die was independently rotated through a 90.degree. angle about a vertical axis different than the bend die rotation axis. That is, after the upper and lower sections were rotated together about a first vertical axis to form a bend, the upper and lower sections of the bend die were split and then the upper section was independently rotated about a different axis. The independent rotation caused a portion of the upper section to be displaced laterally to vacate a region directly above the lower section. Then, when the tube was rotated to form a zigzag and/or moved forwardly to position for the next bend, a portion of the tube was permitted to pass through the vacated region. With such arrangement, the minimum bend angle between successive bends was less restricted than without rotating the upper section of the split bend die. For example, without rotating the upper section of the split bend die, the angle between successive bends or segments in the zigzag configuration had to be relatively large such as 108.degree. to clear the upper section of the bend die. However, by rotating the upper section out of the way, smaller bend angles such as, for example, 60.degree. could be formed. As a result, heat exchanger segments could be densely packed in a relatively low profile furnace.
One drawback of the above described arrangement is that a complicated mechanism using nonstandard designs or practices is generally required to rotate the upper section of the bend die independently. During a bending operation, the upper and lower bend die sections are locked together and rotated about a first axis. It then becomes a complicated mechanical task to lift the upper section and rotate it about a second axis.